Ty asked Lisa, "What shall we do for the next..." He looked at the clock. "Hour and a half?"
"Do you really need to ask?" Lisa said, as she took off the shorts and bikini top again. Her nipples reappeared, though they were transparent, and she grew two more pairs of arms and breasts. Almost immediately, she swore. Apparently the fish that liked poking her nipples from the inside had wasted no time, and was at it again. "I wonder," she said, and turned her nipples into pussies, still transparent. "Will the fish poke at those, too?"
"Oh, I want one other thing," she said, and she made a copy of herself in mattress topper form. "Your plan for mattress topper form to experience the sexual pleasure of anyone on atop it was absolutely inspired!" She grew another pair of arms, and breasts with pussies, as she pulled Ty onto her other self.
Ty gave himself more arms and cocks, to match hers. They had very enthusiastic sex, and after, Ty said, "I think when we do that, all the rapid sloshing of the water in you back and forth might hurt the fish. Maybe you should transform out of aquarium form before we fuck."
Lisa looked devestated. "Oh, I never thought of that. I wouldn't want to hurt my fish! But it's a big turn-on for me, fucking in this form."
Ty said, "Well, maybe I can do something about that." He concentrated a bit, and Lisa felt something going on in her aquarium part, but she didn't know what it was.
"What did you just do?" she asked.
"I changed your aquarium part to increase the water's inertia when subjected to sudden acceleration," Ty explained. "Most of the time, the water inside you will slosh around just like normal. But if you move suddenly, including when we fuck, the water won't move so much, and the fish won't be subjected to as much of the acceleration. Effectively, that makes the water behave somewhat as if it was anti-thixotropic, like oobleck. But it's not exactly like that, because that would probably hurt the fish, too."
"Wow! You've changed the laws of physics, just to protect my fish!" she said, and kissed him.
"My pleasure," he said.
About that time, there was a knock on his bedroom door. "I'll go get that," Ty said, getting up and hastily getting dressed. "It might be Mark. Maybe you should merge, and change back to normal but aquarium form, and get dressed." He went through the portal door, closing it behind him, and opened his bedroom door. As expected, it was Mark.
Mark said, "Your mother asked me to have a look at Lisa's transformation, and decide whether she can have it in our house, outside your bedroom. Is she here? Is it OK if I see her?"
"Absolutely, Dad," Ty said. "We've been expecting you." He knocked on the portal door, and Lisa said, "Come on in."
Mark was absolutely astounded to see Lisa. "June told me what to expect, but seeing it is a whole 'nother thing. That's incredible. You're... beautiful! I mean, you're beautiful always, but now... beautiful in an additional, amazing way."
"Thank you, Mark," Lisa said.
"So what do you think, Dad?" Ty asked. "Is it OK for her to have this form in our house?"
"Well, it doesn't offend your mother, or me," Mark said. "I'm not sure what Dustin's reaction will be, but Lisa, you should probably expect him to stare at you. Hell, I'll probably stare at you. So I apologize in advance for that."
"Don't worry about it," Lisa said. "I fully expect to be stared at. That's kind of the point."
"OK," Mark said, "we'll allow it."
"Thank you!" Lisa said, and stood on her tiptoes to kiss his cheek. Then she hugged Ty, and kissed him.
"June said that dinner will be in about twenty minutes," Mark said, as he turned to leave.
"Time enough for a quickie," Lisa whispered to Ty.
"You're going to wear me out," he facetiously complained. "But I guess we do need to check whether the changes I made protect your fish."
She was in too big a hurry to make a mattress topper copy of herself this time. They undressed, and went right at it, though Ty was a little distracted trying to watch her fish as the two of them fucked with wild abandon. Just before they came, he concluded that the changes worked.